South Korea's military said it scrambled stealth jets on Friday after detecting the mobilization of 180 North Korean warplanes.

There were no immediate reports of clashes.

The Joint Chiefs of Staff of South Korea reported that 80 aircraft, including F-35A stealth fighters, were scrambled after North Korean aircraft were spotted at many locations north of the "tactical action line."

The virtual line drawn north of the physical border serves as the basis for air defence operations in South Korea, according to a South Korean official.

None of the aircraft crossed the "tactical action line."

Pyongyang unhappy with South Korea-US air drills

Meanwhile, over 240 planes are taking part in the Vigilant Storm air drills with the US. These planes continued their drills, the South Korean military said.

Pyongyang had earlier launched roughly 80 artillery rounds in retaliation for Seoul's joint military exercises with the United States.

The maneuvers came after Pyongyang launched dozens of missiles this week, drawing condemnation from the US, Japan and South Korea.

On Wednesday, North Korea test fired at least 23 missiles, and then launched three more into the Sea of Japan on Thursday, including an intercontinental ballistic missile that triggered evacuation warnings in Japan.

South Korea and the United States retaliated to this string of launches by prolonging the Vigilant Storm exercises, which infuriated Pyongyang.

The North had threatened reprisals over extension of the drills.